1. Field of the Invention
The present invention relates to a method for measurement (measurement method) of a magnetic field gradient. In particular, the present invention relates to a measurement method of a magnetic field gradient of a recording magnetic field in a recording medium, the recording magnetic field being generated by a magnetic head that writes information to the recording medium using a thermally-assisted magnetic recording system.
2. Description of the Related Art
In recent years, higher recording density of a hard disk drive (HDD) has been demanded, and a perpendicular magnetic recording system has been adopted as its recording system. In the perpendicular magnetic recording system, a magnetization direction of a recording bit (magnetic domain) is perpendicular to a recording medium. Thereby, the magnetic domain size is decreased and higher recording density is realized.
Generally, when the magnetic domain size is miniaturized, along with a volume decrease in magnetic particles that configure the recording medium, a problem of thermal stability of a recording magnetization occurs. In other words, due to “thermal fluctuation” of the magnetization, information recorded in the recording medium may be lost. The same things occur also in the perpendicular magnetic recording system. As a method to resolve this problem of thermal stability, it can be considered to use a material having large magnetic anisotropy for the recording medium. However, in order to write information (that is, to perform magnetization reversal) to such recording medium, an extremely large recording magnetic field (writing magnetic field) generated by a magnetic head is needed. This may not allow stable writing to be performed in some occasions.
In order to perform stable writing to a recording medium formed of a material having large magnetic anisotropy, a recording system called as a thermally-assisted magnetic recording system has been proposed. According to the recording system, by heating a portion of the recording medium to which information should be written, coercive force of the portion can be decreased and a recording magnetic field needed for magnetization reversal can be decreased. As a result, stable writing becomes possible.
For further increase in recording density, it is necessary to narrow a magnetization reversal width (magnetization transition width) in a track width direction. In the perpendicular magnetic recording system, a magnetization transition width 6 is expressed as follows by using a coercive force distribution ΔHC of the recording medium and a magnetic field gradient dHh/dx of a writing magnetic field, which is generated by the magnetic head, in the recording medium.
                    δ        =                              Δ            ⁢                                                  ⁢                          H              C                                            (                                          ⅆ                                  H                  h                                                            ⅆ                x                                      )                                              (        1        )            
On the other hand, a magnetization transition width 6 in the thermally-assisted magnetic recording system is given by
                    δ        =                                            Δ              ⁢                                                          ⁢                              H                C                                                                    (                                                                            ⅆ                                              H                        C                                                                                    ⅆ                      T                                                        ·                                                            ⅆ                      T                                                              ⅆ                      x                                                                      )                            +                              (                                                      ⅆ                                          H                      h                                                                            ⅆ                    x                                                  )                                              .                                    (        2        )            
Herein, dHC/dT is a temperature gradient of coercive force of the recording medium, and dT/dx is a temperature gradient of the recording medium.
In the thermally-assisted magnetic recording system, normally, a method of heating the recording medium by irradiating the recording medium with light of near-field light is used. In this method, a steep temperature gradient can be generated on a surface of the recording medium. Therefore, the magnetization transition width 6 is expected to be narrowed by combining a recording medium having a large temperature gradient of coercive force. Also in this point, the thermally-assisted magnetic recording system is advantageous as compared with the perpendicular magnetic recording system.
For design of a magnetic head and a recording medium that realize higher recording density, it is important to know a magnetization transition width itself. According to the above-described expression, the magnetization transition width can be calculated using a magnetic field gradient (recording magnetic field gradient) of a writing magnetic field generated by the magnetic head. However, because a measurement (calculation) method of the recording magnetic field gradient has not been established, actual measurement thereof is difficult. In the present state, the recording magnetic field gradient is treated as a prediction value by a calculator simulation. However, it is not more than a prediction value, and it cannot be said that the prediction value is sufficient to obtain an actual recording magnetic field gradient.